Dies for RFID devices and sensor applications

ABSTRACT

Deep reactive ion silicon etching of a device wafer, laser-induced release of individual dies, and individual placement of the dies on flexible substrates facilitate formation of circuits having relatively large external inductors for powering devices such as RFID devices. Small flexible tabs enable die-inductor connection. The absorption properties of both an adhesive layer and an ablation layer may be employed to facilitate debonding of individual dies from a glass handler without damaging integrated circuits associated with the dies. Die packs including cavities for accepting individual dies facilitate fabrication processes using dies having small dimensions.

FIELD

The present disclosure relates to semiconductor device fabrication andRFID and sensor devices obtained thereby.

BACKGROUND

Semiconductor device fabrication includes the preparation of a wafer forintegrated circuit packaging. A wafer can, for example, be mounted to apolymeric substrate having an adhesive film thereon. Once mounted,individual dies can be obtained from a wafer including a large number ofintegrated circuits using a singulation process. The individual diesremain in place on the substrate.

RFID (radio frequency identification) devices are employed foridentification and tracking purposes through the use of electromagneticfields. Tags including such devices may be powered by electromagneticinduction. RFID tags including electrically insulating, flexiblesubstrates including antennas electrically coupled to integrated circuitchips have been disclosed. Radio frequency identification, or RFID, usesradio waves to automatically identify people or objects. The most commonmethod of identification is to store a serial number that identifies aperson or object, and perhaps other information, on a microchip that isattached to an antenna. The antenna enables the chip to transmit theidentification information to a reader. The reader converts the radiowaves reflected back from the RFID tag into digital information that canthen be passed on to computers that can make use of it. Radio FrequencyIdentification (RFID) is becoming an important identification technologyfor tracking objects such as packages, merchandise, luggage and thelike. RFID systems provide additional identification functions not foundin more conventional identification technologies such as barcodes.Unlike a barcode, RFID tags do not necessarily need to be within line ofsight of the reader, and may be embedded in the tracked object.

3D chip technologies, including both 3D ICs (integrated circuits) and 3Dpackaging, may utilize through-silicon vias (TSV). A TSV is a verticalinterconnect access (VIA) in which a connection passes entirely througha silicon wafer or die. TSVs can allow more tightly integratedstructures than edge wiring.

Temporary wafer bonding/debonding includes the act of attaching asilicon device wafer to a substrate or handling wafer so that it can beprocessed, for example, with wiring, pads, and joining metallurgy, whileallowing the wafer to be thinned. Debonding is the act of removing aprocessed silicon device wafer from a substrate or handling wafer. Someexisting approaches for temporary wafer bonding/debonding involve theuse of an adhesive layer placed directly between a silicon device waferand a handling wafer. When the processing of the silicon device wafer iscomplete, the silicon device wafer may be released from the handlingwafer by various techniques such as by exposing the wafer pair tochemical solvents delivered by perforations in the handler, bymechanical peeling from an edge initiation point or by heating theadhesive so that it may loosen to the point where the silicon devicewafer may be removed by sheering. The blanket release of a handler canalternatively be conducted using a scanning laser at 355 nm. Commonlyassigned US20140106473, which is incorporated by reference herein,describes a method for effecting full wafer release of a handler.

SUMMARY

Principles of the present disclosure provide an exemplary fabricationmethod that includes obtaining a structure comprising a device wafer.The device wafer includes a device side including a plurality ofintegrated circuits. The structure further includes a UV-transmissivehandler and a bonding layer that bonds the handler to the device wafer.The exemplary fabrication method further includes applying a resistlayer to the device side of the device wafer, patterning the resistlayer, and singulating the device wafer using deep reactive ion etching,thereby forming a plurality of dies separated by trenches extendingcompletely through the device wafer. The patterned resist layer isremoved. A die pack including a plurality of die cavities is providedand the singulated dies are aligned with the dies cavities. At leastpart of the bonding layer is subjected to UV radiation through theUV-transmissive handler. Fluence of selected energy is released in aspot within the bonding layer, the spot having dimensions correspondingto the dimensions of the dies, thereby causing release of an individualone of the dies from the UV-transmissive handler into one of the diecavities.

A second exemplary method includes obtaining a structure including adevice wafer including a device side comprising a plurality ofintegrated circuits, a UV-transmissive handler, and a bonding layer thatbonds the UV-transmissive handler to the device wafer, the device waferhaving a thickness of 200 μm or less. A resist layer is applied to thedevice side of the device wafer and is patterned. The method furtherincludes singulating the device wafer using deep reactive ion etching,thereby forming a plurality of dies separated by trenches extendingcompletely through the device wafer. The patterned resist layer isremoved and a die pack including a plurality of die cavities isprovided. Each of the die cavities is configured for accepting anindividual one of the dies. The dies are aligned with the die cavitiesand at least part of the bonding layer is subjected to UV radiationthrough the UV-transmissive handler, thereby causing selective releaseof one or more of the dies from the UV-transmissive handler into one ormore of the cavities within the die pack.

An exemplary RFID device includes a flexible, electrically insulatingsubstrate having a thickness between 25-100 μm, the substrate includinga main portion and an integral tab extending from the main portion. Thetab is foldable with respect to the main portion of the substrate. Anantenna is integral with the main portion of the substrate. Anelectrical contact is integral with the tab and is electricallyconnected to the antenna. A RFID die including an integrated circuit ismounted to the main portion of the substrate and electrically connectedto the antenna. The die has a thickness of 200 μm or less and a width of200 μm or less. The electrical contact is positioned to contact a topsurface of the die when folded with respect to the main portion of thesubstrate.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Fabrication methods as disclosed herein can provide substantialbeneficial technical effects. For example, one or more embodiments mayprovide one or more of the following advantages:

-   -   Facilitates production of individual dies having very small        dimensions and individual placement on a substrate;    -   Enables production of RFID tags having small dimensions and fold        tabs configured to contact the top surface of a dielet;    -   Precision etched die pack enables handling and picking of        dielets where small dielet dimensions effectively preclude        conventional processing.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional illustration showing an assemblyincluding a silicon wafer bonded to a glass handler;

FIG. 2 is a schematic sectional illustration showing the assembly ofFIG. 1 following reduction in thickness of the silicon wafer;

FIG. 3 is a schematic sectional illustration showing the assembly ofFIG. 2 following bonding of a second glass handler to the silicon wafer;

FIG. 4 is a schematic sectional illustration showing the assembly ofFIG. 3 following removal of the original glass handler and cleaning ofthe device side of the silicon wafer;

FIG. 5 is a schematic sectional illustration showing the assembly ofFIG. 4 following the application and patterning of a photoresist layeron the device side of the silicon wafer;

FIG. 6 is a schematic sectional illustration showing the assembly ofFIG. 5 following deep reactive ion etching (DRIE) of the silicon wafer;

FIG. 7 is a schematic sectional illustration showing the assembly ofFIG. 6 mounted to a tape frame;

FIG. 8 is a schematic sectional illustration showing the assembly ofFIG. 6 aligned with a die pack mold;

FIG. 9 is a schematic perspective view showing the targeted release ofselected dies from a glass handler;

FIG. 10 is a schematic illustration of an assembly for releasingindividual dies from a glass handler;

FIG. 11 is a schematic illustration showing an assembly aligned with adie pack containing individual dies;

FIG. 12 is a schematic illustration showing the assembly of FIG. 11depositing selected individual dies on packages mounted to a substrate;

FIGS. 13A and 13B show schematic top and side views of a deviceincluding a dielet on a flexible substrate.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure provide approaches forthe fabrication of RFID devices using thin silicon wafers subjected todeep reactive ion etch to form individual dies that are releasable byablating a release layer using a laser. The laser used may be anultraviolet (UV) laser, for example, a 355 nm laser, a 351 nm laser or a308 nm laser. The 355 nm wavelength is attractive in some embodimentsdue to the availability of robust and relatively inexpensivediode-pumped solid-state (DPSS) lasers.

The bonding of the silicon device wafer 22 to the handling wafer 28includes the use of both an adhesive layer and a distinct release layerin one or more embodiments. In FIGS. 1-8, the bonding layer 26 mayaccordingly include both an adhesive layer and a distinct release layer.According to one approach for such bonding, the release layer may be anultraviolet (UV) sensitive ablation layer and it may be applied to thehandling wafer, which is a UV-transmissive glass handler in someexemplary embodiments. The UV ablation layer may then be cured. Thebonding adhesive that forms the adhesive layer may be applied to eitherthe glass handler or the silicon device wafer. The UV ablation layer iscomprised of a material that is highly absorbing at the wavelength ofthe laser used in debonding. The material may also be opticallytransparent in the visible spectrum to allow for inspection of theadhesive bonded interface. Both the UV ablation layer and the adhesivelayer are chemically and thermally stable so that they can fullywithstand further processing of the resulting structure. In one or moreexemplary structures, the ablation layer has a thickness between 0.1-0.5μm. The adhesive layer has a substantially greater thickness of between1-100 μm.

An exemplary fabrication method begins with UV ablation material beingapplied e.g. by spin coating onto the glass handler 28. The glasshandler with UV ablation material spin-coated thereon is soft-baked toremove any solvent. Spin coating parameters may depend on the viscosityof the UV ablation layer, but may fall in the range from approximately500 rpm to approximately 3000 rpm. The soft-bake may fall in the rangefrom approximately 80° C. to approximately 120° C. The temperature ofthe final cure may fall in the range from 200° C. to 400° C. Forstrongly UV-absorbing or UV-sensitive materials, very thin final layerson the order of approximately 10001 to approximately 2000 Å thick may besufficient to act as release layers. Some organic planarizing layers(OPLs) and organic dielectric layers (ODLs) have such properties. Inother embodiments, a dye is incorporated within the polymeric materialcomprising the ablation layer to impart the required UV-absorbingproperties. Exemplary dyes that can be employed in one or moreembodiments include 9-anthracenecarboxylic acid and benzanthrone addedat a weight percentage of at least ten percent to any non-absorbingmaterial capable of forming a film from solution such aspolymethylmethacrylate (PMMA). The incorporation of dyes is discussedfurther below with respect to the adhesive layer. Some exemplary ODLmaterials are spin applied to glass and cured in a nitrogen environmentat 350° C. for approximately one hour to produce a film. Such a film maybe optically transparent throughout the visible spectrum, but stronglysensitive to decomposition in the UV wavelength range below about 360nm, and may be fully and cleanly ablated using common UV laser sourcessuch as an excimer laser operating at 308 nm (e.g. XeCl) or 351 nm (e.g.XeF) or a diode-pumped tripled YAG laser operating at 355 nm.

Referring to FIG. 1, a structure 20 is obtained that includes a siliconwafer 22 bonded to a glass handler 28. The silicon wafer includes adevice layer 24. The device layer includes a plurality of integratedcircuits. In some embodiments, each of the integrated circuits isidentical. A bonding layer 26, which may comprise multiple layers asdescribed above, adjoins the device side of the silicon wafer and theglass handler. The thickness of the silicon wafer is reduced by grindingor other suitable process to obtain the structure 30 shown in FIG. 2. Insome embodiments, the thickness of the silicon wafer is reduced to afinal thickness of about fifty micrometers (50 μm) to form a silicondevice wafer.

A second glass handler 28′ is bonded to the structure 30 to obtain thestructure 40 shown in FIG. 3. The bonding layer 26 used to secure thesecond glass handler 28′ to the exposed surface of the silicon wafer 22may be a multiple layer including an adhesive layer and a UV-sensitiverelease layer as discussed above. The two bonding layers 26 in thestructure 40 may or may not be identical in composition.

The original glass handler 28 is removed from the structure 40 to obtainthe structure 50 shown in FIG. 4. Laser debonding to release the glasshandler at the ablation layer interface may be performed using any oneof a number of UV laser sources including excimer lasers operating at308 nm (e.g. XeCl) or 351 nm (e.g. XeF) as well as diode-pumped(tripled) YAG laser operating at 355 nm or diode-pumped (quadrupled) YAGlaser operating at 266 nm. Excimer lasers may be more expensive, mayrequire more maintenance/support systems (e.g. toxic gas containment)and may have generally have very large output powers at low repetitionrates (e.g. hundreds of Watts output at several hundred Hz repetition).UV ablation thresholds in the materials specified here may require100-150 milliJoules per square cm (mJ/sqcm) to effect release. Due totheir large output powers, excimer lasers can supply this energy in arelatively large area beam having dimensions on the order of tens ofsquare millimeters area (e.g. 0.5 mm times 50 mm line beam shape). Dueto their large output power and relatively low repetition rate, a laserdebonding tool which employs an excimer laser may include a movable x-ystage with a fixed beam. Stage movement may be on the order of ten tofifty mm per second. The wafer pair to be debonded may be placed on thestage and scanned back and forth until the entire surface had beenirradiated.

An alternative laser debonding system may be created using a lessexpensive, more robust and lower power solid-state pumped tripled YAGlaser at 355 nm by rapidly scanning a small spot beam across the wafersurface. The 355 nm wavelength laser may compare favorably to thequadrupled YAG laser at 266 nm for two reasons: 1) output powers at 355nm are typically two to three times larger than at 266 nm for the samesized diode laser pump power, and 2) many common handler wafer glasses(for example, Schott Borofloat 33) are about ninety percent or moretransmissive at 355 nm but only about fifteen percent transmissive at266 nm. Since eighty percent of the power is absorbed in the glass at266 nm, starting laser powers may be about six times higher to achievethe same ablation fluence at the release interface. There is accordinglyrisk of thermal shock in the glass handler itself.

An exemplary 355 nm scanning laser debonding system may include thefollowing: 1) a Q-switched tripled YAG laser with an output power of 5to 10 Watts at 355 nm, with a repetition rate between 50 and 100 kHz,and pulsewidth of between 10 and 20 ns. The output beam of this lasermay be expanded and directed into a commercial 2-axis scanner,comprising mirrors mounted to x and y galvanometer scan motors. Thescanner may be mounted a fixed distance above a fixed wafer stage, wherethe distance would range from 20 cm to 100 cm depending on the workingarea of the wafer to be released. A distance of 50 to 100 cm mayeffectively achieve a moving spot speed on the order of 10meters/second. An F-theta lens may be mounted at the downward facingoutput of the scanner, and the beam may be focused to spot size on theorder of 100 to 500 microns. For a six watt output power laser at 355nm, at 50 kHz repetition and 12 ns pulsewidth, a scanner to waferdistance of 80 cm operating at a raster speed of 10 m/s, the optimalspot size may be on the order of 200 microns, and the required about 100mJ/sq. cm ablation fluence may be delivered to the entire wafer surfacetwice in about thirty seconds (for example, using overlapping rows). Theuse of overlapping rows where the overlap step distance equals half thespot diameter (e.g., 100 microns) may ensure that no part of the waferis missed due to gaps between scanned rows and that all parts of theinterface see the same total fluence. US 2014/0106473 discloses anexemplary method of effecting full wafer and handler release.

An exemplary approach for performing handler wafer bonding and debondingin accordance with exemplary embodiments includes applying the releaselayer to the handler while an adhesive layer may be applied to thedevice wafer. However, according to other exemplary approaches, therelease layer may be applied to the handler and then the adhesive layermay be applied to the release layer. The release layer is interposedbetween the glass handler and the adhesive. Thereafter, the device wafermay be bonded to the handler such that the release layer and theadhesive layer are provided between the device wafer and the handler.The bonding may include a physical bringing together of the device waferand the handler under controlled heat and pressure in a vacuumenvironment such as offered in any one of a number of commercial bondingtools. After the device wafer has been successfully bonded to thehandler, desired processing may be performed. Laser ablation is employedto allow separation of the device wafer 22 from the original glasshandler 28 along the plane of the ablation (release) layer. For pulsesin the range of 10-20 nanoseconds, ablation may include photothermal,photomechanical and/or photochemical ablation of the ablation layer. Thethinned device wafer 22 is substantially opaque to UV radiation and willaccordingly block such radiation during release of the original glasshandler 28. The device wafer is then cleaned to remove residualadhesive. The bond between the second glass handler and the device waferis unaffected. The ablation layers within the two bonding layers may ormay not be comprised of the same materials, but are preferably the same.A thin silicon device layer can alternatively be obtained by controlledspalling of the silicon wafer prior to bonding it to a glass handler.Controlled spalling is performed using a flexible handle layer (e.g.Kapton tape) attached (e.g. bonded) to a metal (e.g. nickel) stressorlayer to cause a fracture in a substrate along a desired plane.Exemplary controlled spalling techniques are disclosed in US Pub. Nos.2010/0307572 and 2011/0048517, both of which are incorporated byreference herein. If spalling is employed, the spalled portion of thesilicon wafer would be bonded to a glass handler. The flexible handleand metal stressor layers would then be removed to obtain a structure asshown in FIG. 4.

Referring to FIG. 5, a resist layer 32, typically ranging from two (2)to ten (10) microns in thickness, is applied to the device side of thesilicon wafer 22. A reverse tone resist is employed in some embodiments.The resist layer is patterned to obtain the structure 60 asschematically illustrated. The distance between the trenches formed inthe resist is approximately the size of the dies to be obtainedfollowing singulation. The dies to be obtained in some embodiments aretwo hundred microns or less on a side and one hundred microns or less inthickness. Dies within such a size range are sometimes referred to asdielets.

Deep reactive ion etching (DRIE) is employed to obtain multiple dies 22′from the device wafer 22, each comprising an integrated circuit 24′.Trenches having widths of about ten microns (10 μm) are formed in someembodiments. The highly anisotropic etch etches silicon preferentiallyand causes the walls of the trenches to be substantially vertical. Thetrenches extend completely through the silicon device wafer 22. If oxideis on the surface, a different etchant may be employed initially toremove the oxide followed by etching of the silicon. An exemplaryresulting structure 70 is schematically illustrated in FIG. 6. Thestructure comprises the second glass handler 28′, an array of dies 22′and associated integrated circuits 24′, and a bonding layer 26 thatbonds the dies to the handler 28′. Dies having small dimensions can beobtained using the techniques described herein. Die thicknesses in therange between 30-200 μm are obtained in some embodiments, and arepreferably 50 μm or less. The dies obtained in some embodiments are 200μm or less on each side. In an exemplary embodiment, dies are obtainedhaving one side of 200 μm or less and another side of 50 μm or less. Inembodiments where the dies contain through silicon vias, the conductordiameter is about five microns in some embodiments. Such vias can beformed of plated copper, CVD-deposited tungsten, or other suitableelectrical conductor. The bonding layer can be subjected to UV radiationto release the dies either individually or in groups, as discussedfurther below.

Device fabrication can be facilitated by releasing selected dies orgroups of dies to a tape having an adhesive surface or into a precisiondie pack. Laser release need not involve scanning the beam uniformly,but rather it is directed to selectively release thin dielets in smallgroups, as a single row or column, or even as discrete dielets in someembodiments. Referring to FIG. 7, the exemplary structure 70 is mountedto a tape frame 34 such that the dies 22′ adjoin the tape. As discussedabove, the ablation layer contained within the bonding layer 26 ischosen to be highly absorptive in the ultraviolet spectrum of interest,namely between 308 nm and 355 nm. In some embodiments, about eighty toninety percent of the laser fluence is absorbed by the ablation layer.Such absorption enables wafer separation as the ablation layerdisintegrates. The remainder of the fluence penetrates into the adhesivelayer. In some embodiments of the exemplary structure 70, the adhesivelayer is also capable of absorbing fluence at the desired wavelengths(308-355 nm). By providing an ablation layer and an adhesive layer thatboth have absorption properties, only a negligible amount of thestarting fluence is allowed to reach the surfaces of the dies thatinclude the integrated circuits 24′. Release of the dies 22′ to the tapeis facilitated in some embodiments by an optical scanner and a pulsedlaser that provide a high speed targeted release of collections of dies22′ or individual dies.

In some embodiments, dielets obtained following singulation have suchsmall dimensions that handling them and picking them from a tape becomesimpractical. Rather than releasing the dies 22′ to a tape frame, theyare alternatively released to a die pack mold 36 in accordance with afurther exemplary embodiment, as schematically illustrated in FIG. 8.The die pack mold in one exemplary embodiment includes a silicon waferthat is subjected to patterning and etching (for example, DRIE). The topsurface of the wafer includes an array of cavities 38 that havedimensions corresponding to those of the dies 22′ to be depositedtherein. The cavity widths and lengths are substantially the same as thewidths of the dies 22′. Cavity depths are the same or somewhat less thanthe depths of the dies 22′ so that surfaces of the dies remainaccessible once deposited in the die cavities. Silicon partitionsdelineate the cavities. As schematically illustrated in FIG. 9, UVradiation 42 is emitted from a UV source, passes through the handler28′, and penetrates the UV-sensitive bonding layer 26. The spot 44 sizeof the laser-generated radiation substantially matches the dimensions ofthe dies in some exemplary embodiments, allowing local release from thesecond glass handler 28′. The dies 22′ are accordingly transferred toeither tape or into the cavities formed in a die pack mold. As indicatedabove, groups of dies are transferred in some embodiments and individualdies are transferred in other embodiments.

FIG. 10 shows an exemplary system for wafer or die level release. Such asystem can accordingly be employed for releasing the glass handler 28from the device wafer 22, as discussed above with respect to FIG. 4, ordies 22′ from the exemplary structure 70 described above with respect toFIG. 6. In one exemplary embodiment, a Q-switched triple YAG laser 82,an adjustable beam expander, and a scanner 92 are operatively associatedwith a computer 86 and a control box 84. An F-Theta scan lens 94 havingan appropriate focal length is operatively associated with the scanner92.

Penetration depth is a measure of the depth electromagnetic radiationcan penetrate into a material, specifically the depth at which theintensity of the radiation falls to 1/e or about 36.8% of its originalvalue at the substrate surface. Penetration depth δ_(p) is generally afunction of wavelength for a given material. Intensity decreases as afunction of thickness measured in penetration depths. For example, whileintensity is about 36.8% of the original intensity at one penetrationdepth, it is only about 13.5% of the original intensity at twopenetration depths and about five percent at three penetration depths.Referring again to FIG. 9, UV light 42 is directed to theUV-transmissive handler 28′. In the exemplary embodiment, only aboutfive to fifteen percent of the fluence at the surface of the handlerenters the adhesive layer portion of the bonding layer 26, due largelyto the absorption by the ablation layer of the bonding layer 26 thatadjoins the glass handler 28′. The adhesive layer allows less than abouttwo percent of the original fluence to exit towards the device wafer 22.In the exemplary embodiments, the penetration depth of the ablationlayer is between about 0.1-0.2 μm while the penetration depth of thethicker adhesive layer is between two and twenty micrometers. Theablation layer in one or more embodiments is on the order of 0.2-0.3 μmin thickness. This confines the laser pulse energy (about one hundredmJ/cm² for about ten nanoseconds duration in some embodiments) to a verythin zone adjacent to the handler to achieve complete release atreasonable fluence.

Certain high-temperature polymer adhesives based on polyimide absorb UVradiation in the wavelength range between 350 nm and 300 nm and comprisethe adhesive layer in some embodiments. Thus, the amount of residual UVfluence reaching the active wafer surface can vary depending on thethickness uniformity of the original ablation layer and the opticalproperties and thickness of the adhesive layer below. Coating defects inthe ablation layer may lead to yield loss unless there is additionalfiltering of the UV pulse over the substantially greater thickness ofthe adhesive layer. The adhesive layer employed, as combined with theablation layer, have the necessary optical properties to help preventlaser induced damage that could result from an appreciable amount of theablation pulse reaching the active wafer or die surface where it couldinteract with materials such as polyimide or PECVD silicon nitride(SiN_(x)) passivation layers.

In accordance with one or more embodiments, a multi-layer debondingstructure includes two layers, namely the ablation layer and theadhesive layer, having absorption properties and thicknesses that ensurethat no more than a negligible amount of the ablation fluence is allowedto reach the device wafer surface or the die surfaces. By specifying therequired UV absorption requirements of both the ablation layer and theunderlying adhesive, debonding can be safely conducted without asubstantial risk of causing laser induced damage. In exemplaryembodiments, the ablation layer has a thickness of at least twopenetration depths, and preferably between two and four penetrationdepths. The adhesive layer has a thickness of at least one penetrationdepth and preferably between one and two penetration depths. Thepenetration depth of the ablation layer is between 0.1 and 0.2 micronsin one or more embodiments while the penetration depth of the adhesivelayer is between two and twenty microns in one or more embodiments.

In some embodiments, the adhesive layer has intrinsic optical absorptionproperties in the desired range of wavelengths. An exemplary commercialadhesive which readily absorbs UV laser radiation in the wavelengthrange from 300 nm to 360 nm would be the polyimide-based product by HDMicrosystems called HD-3007 Adhesive. This commercial adhesive is anon-photodefinable polyimide precursor designed for use as a temporaryor permanent adhesive in 3D packaging applications. It exhibitsthermoplastic behavior after cure and during bonding at moderatetemperature and pressure. Thermoplastic adhesives having base materialsthat do not have intrinsic optical absorption at the laser wavelength(s)desired, or have insufficient optical absorption properties, aremodified in some embodiments by the addition of fine nanoparticles.Suspensions of the nanoparticles can be added in amounts which, whenuniformly dispersed throughout the adhesive, lead to the approximationof a neutral density filter which scatters a known percentage of theincoming laser pulse with little dependence on wavelength. Exemplarynanoparticles include aluminum and alumina nanoparticles. In otherexemplary embodiments, dyes are added to thermoplastic adhesives that donot exhibit the desired absorption properties. Some dyes are known toabsorb in the laser wavelengths employed in one or more embodiments. Asdisclosed, for example, in U.S. Pat. No. 5,169,678, which isincorporated by reference herein, various dyes can be added to polymericmaterials to affect the absorbance thereof. In some examples, thepolymer is melted and the dye is added to the polymer melt. In otherexamples, the dye is diffused or dissolved into the polymer using asolvent. Even distribution of the dye is obtained in some embodiments.Dyes such as p-phenylazophenol, N-pmethoxybenzylidene-p-phenylazoaniline, dihydroxyanthraquinone and betacarotine are among those that may be employed to provide absorbance inthe UV range. Such dyes may be used as formulated in some embodiments orwith substitutions to adjust the absorbance frequencies. Excitonproducts such as “DPS” (CAS 2039-68-1) and “Bis MSB” (CAS 13280-61-0)are other exemplary materials that can be employed within polymers toprovide absorbance in the UV range in one or more embodiments. Furtherexemplary dyes that can be employed in one or more embodiments include9-anthracenecarboxylic acid and benzanthrone.

An exemplary coating process for either the thin ablation layer or theHD-3007 adhesive includes dispensing of a few ml of the material, spinapplying at between 1000 and 3000 rpm for sixty seconds, baking at about110° C. to drive off the solvent, and curing on a hotplate or in anitrogen oven at about 350° C. for ten minutes. A specific bondingrecipe for HD-3007 adhesive includes aligning the adhesive-coated waferto the handler, holding them apart by a small distance using spacers,and introducing the wafer pair into a chamber where vacuum would bepulled, such that the space between them is fully evacuated. Thetemperature would ramp up to above 100° C. to help de-gas the adhesive,and the spacers would be removed to place the wafer and handler incontact. Heating plates above and below would ramp up to a final bondingtemperature of between 300° C. and 350° C., and a pressure of about 8000mbar would be applied to the pair for five minutes to effect bonding.The pair would be held under pressure as the plates ramped back down tobelow the glass transition temperature Tg.

The tailored release layer comprising part of the bonding layer 26 canbe fully ablated to release the device wafer 22 from the original glasshandler 28 or, as shown in FIG. 9, the singulated dies 22′ from thesecond glass handler 28′. The laser 82 in one or more embodimentsoperates at 355 nm and provides 12 nsec pulses at 50 kHz. A largeseparation distance between the laser and substrate allows for therelatively quick scanning of an entire wafer. Smaller separationdistances facilitate the precision releases of individual dies. Arelease fluence of about 100 mJ/cm² in a 100 μm diameter spot requiresabout 1.5 Watts. A typical spot size is between 0.1-0.5 mm.

FIGS. 11 and 12 illustrate further steps in an exemplary fabricationprocess once dies 22′ have been transferred to cavities 38 within asilicon die pack 36 as discussed above with respect to FIG. 8. Suchsteps provide a practical solution for selectively picking dieletshaving very small dimensions and placing them onto electronic packagessuch as RFID flex packages for bonding as discussed further below. Aprecision etched silicon bonding head 92 picks up every nth dieaccording to the needs of the fabrication process. The head 92 includesnozzles 93 having outer diameters that substantially equal the widths ofthe dies 22′. The nozzles apply suction forces to the dies 22′ to allowthe dies to be removed from the cavities 38 as the head 92 is raised.The dies 22′ are transferred to packages 96 by the head 92 as shown inFIG. 12 and the suction pressure is discontinued. The dies are bonded topackages 96. Features of the head 92, such as the nozzles and internalnozzle passages that provide suction, are obtained by DRIE in one ormore embodiments.

FIGS. 13A and 13B show a RFID device 100 including a flexible substrate102 having an integral tab 104 extending therefrom. An antenna 106 iselectrically connected to a RFID chip 110 at or near the center of themain body portion of the flexible substrate in some embodiments. Thewidth of the main body portion is greater than the width of the integraltab, as shown in FIG. 13A. In one or more embodiments, the width of themain body portion is at least twice the width of the tab and may be asmuch as twenty times greater than the width of the tab 104. The antennais formed through metallization of the flexible substrate, including thetab, to provide a generally spiral configuration having one end near thecenter of the flexible substrate and another end terminating at anelectrically conductive contact 108 near the outer end of the tab 104.The antenna occupies the majority of the area that forms the main bodyportion of the substrate 102. In some embodiments, the antenna consistsessentially of copper. Once the metallization process is complete andthe antenna and contact 108 have been formed, the RFID chip 110 isdeposited on the flexible substrate. The fabrication processes describedabove with respect to FIGS. 1-12 may be employed to fabricate the RFIDchip and transfer it to the flexible substrate. In other words, the dies22′ are RFID chips in one or more embodiments and the packages 96 (FIG.12) comprise the flexible substrates 102 having antennas integraltherewith. The tab 104 has a substantially smaller width than that ofthe main body of the flexible substrate. The length of the tab issufficient to allow the tab to overlap the portion of the main body ofthe flexible substrate that includes the RFID chip 110. The contact 108can accordingly engage the top surface of the chip 110 when the tab isfolded, as shown in FIG. 13B. In one or more embodiments, the tab 104has a width between ten microns and two millimeters and a length betweenone hundred microns and four millimeters. Because the chip 110 can beelectrically connected to an external inductor via a tiny flexible loopformed by the tab 104, a very small package is obtainable. A throughsilicon via (TSV) that completes the chip-inductor loop further helpsreduce package size. The flexible substrate comprises electricallyinsulating materials such as Kapton, polyethylene terephthalate (PET),or thin glass in some embodiments. The listed materials should beconsidered exemplary and not limiting.

Given the discussion thus far and with reference to the exemplaryembodiments discussed above and the drawings, it will be appreciatedthat, in general terms, an exemplary fabrication method includesobtaining a structure comprising a device wafer. The device wafer 22includes a device side 24 including a plurality of integrated circuits.The structure further includes a UV-transmissive handler 28′ and abonding layer 26 that bonds the handler to the device wafer. Anexemplary structure 50 is schematically illustrated in FIG. 4. Theexemplary fabrication method further includes applying a resist layer 32to the device side of the device wafer, patterning the resist layer, andsingulating the device wafer using deep reactive ion etching, therebyforming a plurality of dies 22′ separated by trenches extendingcompletely through the device wafer. The patterned resist layer isremoved to obtain a structure 70 as schematically illustrated in FIG. 6.A die pack 36 including a plurality of die cavities 38 is provided andthe singulated dies are aligned with the dies cavities. At least part ofthe bonding layer 26 is subjected to UV radiation through theUV-transmissive handler. Fluence of selected energy is released in aspot 44 within the bonding layer 26, the spot having dimensionscorresponding to the dimensions of the dies, thereby causing release ofan individual one of the dies 22′ from the UV-transmissive handler intoone of the die cavities 38. In some embodiments, the method furtherincludes releasing fluence of the selected energy in a plurality ofspots having dimensions corresponding to dimensions of the dies, therebycausing release of a plurality of individual ones of the dies from theUV-transmissive handler into individual ones of the die cavities. Themethod may further include simultaneously picking individual ones of thedies 22′ from the die cavities and placing the individual ones of thedies picked from the cavities in parallel onto flexible substrates 102including integral antennas 106. On one or more embodiments, each of thedies has a thickness of less than 200 μm and a width of less than 200μm. In some embodiments, the dies 22′ are fifty microns or less inthickness.

A second exemplary method includes obtaining a structure including adevice wafer 22 including a device side 24 comprising a plurality ofintegrated circuits, a UV-transmissive handler 28′, and a bonding layer26 that bonds the UV-transmissive handler to the device wafer, thedevice wafer having a thickness of 200 μm or less. A resist layer 32 isapplied to the device side of the device wafer and is patterned. Themethod further includes singulating the device wafer using deep reactiveion etching, thereby forming a plurality of dies 22′ separated bytrenches extending completely through the device wafer. The patternedresist layer is removed and a die pack 36 including a plurality of diecavities 38 is provided. Each of the die cavities 38 is configured foraccepting an individual one of the dies 22′. The dies are aligned withthe die cavities and at least part of the bonding layer 26 is subjectedto UV radiation through the UV-transmissive handler 28′, thereby causingselective release of one or more of the dies 22′ from theUV-transmissive handler into one or more of the cavities 38 within thedie pack 36. In some embodiments, the exemplary method further includessimultaneously picking individual ones of the dies from the diecavities, as shown in FIG. 11, and placing the individual ones of thedies picked from the die cavities in parallel onto flexible substratesincluding integral antennas, such as shown in FIG. 12. The step ofsubjecting at least part of the bonding layer to UV radiation 42 furtherincludes, in some embodiments, releasing fluence of selected energy in aplurality of spots 44, each spot having dimensions corresponding todimensions of the dies, thereby causing release of a plurality ofindividual ones of the dies from the UV-transmissive handler intoindividual ones of the die cavities. The die pack is obtained in one ormore embodiments by obtaining a silicon substrate, etching the siliconsubstrate to form the plurality of die cavities 38 within the siliconsubstrate, each cavity having width and length dimensions substantiallymatching width and length dimensions of the plurality of dies, each ofthe width and length dimensions being 200 μm or less.

An exemplary RFID device 100 includes a flexible, electricallyinsulating substrate 102 having a thickness between 25-100 μm, thesubstrate including a main portion and an integral tab 104 extendingfrom the main portion. The tab is foldable with respect to the mainportion of the substrate. An antenna 106 is integral with the mainportion of the substrate. An electrical contact is integral with the taband is electrically connected to the antenna. A RFID die including anintegrated circuit is mounted to the main portion of the substrate andelectrically connected to the antenna. The die has a thickness of 200 μmor less and a width of 200 μm or less. The electrical contact ispositioned to contact a top surface of the die when the tab is foldedwith respect to the main portion of the substrate.

Those skilled in the art will appreciate that the exemplary structuresdiscussed above can be distributed in raw form or incorporated as partsof intermediate products or end products such as RFID devices.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof. Terms such as “above” and “below” are used to indicate relativepositioning of elements or structures to each other as opposed torelative elevation. It should also be noted that, in some alternativeimplementations, the steps of the exemplary methods may occur out of theorder noted in the figures. For example, two steps shown in successionmay, in fact, be executed substantially concurrently, or certain stepsmay sometimes be executed in the reverse order, depending upon thefunctionality involved.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the various embodiments has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the forms disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiments were chosen anddescribed in order to best explain the principles of the invention andthe practical application, and to enable others of ordinary skill in theart to understand the various embodiments with various modifications asare suited to the particular use contemplated.

What is claimed is:
 1. An RFID device comprising: a flexible,electrically insulating substrate having a thickness between 25-100 μm,the substrate including a main portion and an integral tab extendingfrom the main portion, the tab being foldable with respect to the mainportion of the substrate; an antenna integral with the main portion ofthe substrate; an electrical contact integral with the tab andelectrically connected to the antenna; a RFID die comprising anintegrated circuit mounted to the main portion of the substrate andelectrically connected to the antenna, the die having a thickness of 200μm or less and a width of 200 μm or less, the electrical contact beingpositioned to contact a top surface of the die when folded with respectto the main portion of the substrate.
 2. The RFID device of claim 1,wherein the die comprises a through silicon via for electricallyconnecting the antenna with the electrical contact.
 3. The RFID deviceof claim 1, wherein the tab has a width between ten microns and twomillimeters and a length between one hundred microns and fourmillimeters.
 4. The RFID device of claim 3, wherein the main portion ofthe flexible substrate has a width that is at least twice the width ofthe tab.